Preparation

Preheat oven to 325°F.

In a bowl whisk together flour, zest, and salt. In another bowl with an electric mixer beat butter, juice, and confectioner's sugar until light and fluffy and beat in flour mixture until a dough is just formed. Pat dough into a disk and chill, wrapped in plastic wrap, 30 minutes, or until firm.

On a lightly floured surface with a floured rolling pin roll dough into a 13- by 12-inch rectangle (about 1/3 inch thick). Cut out cookies with a

3 1/2- to 4-inch heart-shaped cutter dipped in flour and arrange 1 inch apart on baking sheets. Using dough scraps, roll and cut out more cookies in same manner. Bake shortbread in batches in middle of oven 10 to 15 minutes, or until pale golden, and cool completely on baking sheets on racks. Carefully transfer cookies to a platter.

In a double boiler or in a metal bowl set over pan of barely simmering water melt chocolate, stirring until smooth. Pour warm chocolate onto a plate and dip in half of top side of each cookie, transferring cookies, chocolate sides up, to a wax-paper-lined tray. Chill cookies until chocolate is hardened. Cookies keep, chilled, in a airtight container 4 days.